Relational Model CENG 351 1

Transcription

1 Relational Model CENG 351 1

2 Relational Database: Definitions Relational database: a set of relations Relation: made up of 2 parts: Instance : a table, with rows and columns. #Rows = cardinality, #fields = degree (arity). Schema : specifies name of relation, plus name and type of each column. e.g. Students(sid: string, name: string, login: string, age: integer, gpa: real). We can think of a relation as a set of rows or tuples (i.e., all rows are distinct). CENG 351 File Structures and Data Management 2

4 Relational Query Languages A major strength of the relational model: supports simple, powerful querying of data. Queries can be written intuitively, and the DBMS is responsible for efficient evaluation. Precise semantics for relational queries. Allows the optimizer to extensively re-order operations, and still ensure that the answer does not change. CENG 351 File Structures and Data Management 4

5 The SQL Query Language Developed by IBM (system R) in the 1970s Need for a standard since it is used by many vendors Standards: SQL-86 SQL-89 (minor revision) SQL-92 (major revision, current standard) SQL-99 (major extensions) CENG 351 File Structures and Data Management 5

6 Creating Relations in SQL Creates the Students relation. Observe that the type (domain) of each field is specified, and enforced by the DBMS whenever tuples are added or modified. CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL) As another example, the Enrolled table holds information about courses that students take. CREATE TABLE Enrolled (sid: CHAR(20), cid: CHAR(20), grade: CHAR(2)) CENG 351 File Structures and Data Management 6

9 Integrity Constraints (ICs) IC: condition that must be true for any instance of the database; e.g., domain constraints. ICs are specified when schema is defined. ICs are checked when relations are modified. A legal instance of a relation is one that satisfies all specified ICs. DBMS should not allow illegal instances. If the DBMS checks ICs, stored data is more faithful to real-world meaning. Avoids data entry errors, too! CENG 351 File Structures and Data Management 9

10 Key Constraints A set of fields is a (candidate) key for a relation if : 1. Two distinct tuples cannot have identical values in all the fields of a key, and 2. No subset of the set of fields in a key is a unique identifier for a tuple If part 2 false, then it is a superkey. If there is more than one key for a relation, one of the keys is chosen (by DBA) to be the primary key. e.g., sid is a key for Students. (What about name?) The set {sid, gpa} is a superkey. CENG 351 File Structures and Data Management 10

11 Primary and Candidate Keys in SQL Possibly many candidate keys (specified using UNIQUE), one of which is chosen as the primary key. For a given student and course, there is a single grade. CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid)) CENG 351 File Structures and Data Management 11

12 Primary and Candidate Keys in SQL Possibly many candidate keys (specified using UNIQUE), one of which is chosen as the primary key. Students can take only one course, and receive a single grade for that course; further, no two students in a course receive the same grade. CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) ) An IC can prevent storage of instances that arise in practice! CENG 351 File Structures and Data Management 12

13 Foreign Keys, Referential Integrity Foreign key : Set of fields in one relation that is used to `refer to a tuple in another relation. (Must correspond to primary key of the second relation.) Like a `logical pointer. e.g. sid is a foreign key referring to Students: Enrolled(sid: string, cid: string, grade: string) If all foreign key constraints are enforced, referential integrity is achieved, i.e., no dangling references. CENG 351 File Structures and Data Management 13

15 Enforcing Referential Integrity Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students. What should be done if an Enrolled tuple with a nonexistent student id is inserted? (Reject it!) What should be done if a Students tuple is deleted? Also delete all Enrolled tuples that refer to it. Disallow deletion of a Students tuple that is referred to. Set sid in Enrolled tuples that refer to it to a default sid. (In SQL, also: Set sid in Enrolled tuples that refer to it to a special value null, denoting `unknown or `inapplicable.) Similar if primary key of Students tuple is updated. CENG 351 File Structures and Data Management 15

17 Where do ICs Come From? ICs are based upon the semantics of the real-world enterprise that is being described in the database relations. We can check a database instance to see if an IC is violated, but we can not infer that an IC is true by looking at an instance. An IC is a statement about all possible instances! From example, we know name is not a key, but the assertion that sid is a key is given to us. Key and foreign key ICs are the most common; more general ICs supported too. CENG 351 File Structures and Data Management 17

Lecture 6 SQL, Logical DB Design Relational Query Languages A major strength of the relational model: supports simple, powerful querying of data. Queries can be written intuitively, and the DBMS is responsible

EECS 647: Introduction to Database Systems Instructor: Luke Huan Spring 2013 Administrative Take home background survey is due this coming Friday The grader of this course is Ms. Xiaoli Li and her email

Review: Participation Constraints Does every department have a manager? If so, this is a participation constraint: the participation of Departments in Manages is said to be total (vs. partial). Every did

COS 597A: Principles of Database and Information Systems SQL: Overview and highlights The SQL Query Language Structured Query Language Developed by IBM (system R) in the 1970s Need for a standard since

SQL: Queries, Programming, Triggers CSC343 Introduction to Databases - A. Vaisman 1 R1 Example Instances We will use these instances of the Sailors and Reserves relations in our examples. If the key for

Introduction to Database Systems UVic C SC 370 Overview What is a DBMS? what is a relational DBMS? Why do we need them? How do we represent and store data in a DBMS? How does it support concurrent access

Database Security Chapter 21 Introduction to DB Security Secrecy: Users should not be able to see things they are not supposed to. E.g., A student can t see other students grades. Integrity: Users should

The Relational Data Model: Structure 1 Overview By far the most likely data model in which you ll implement a database application today. Of historical interest: the relational model is not the first implementation

The Entity-Relationship Model Chapter 2 Instructor: Vladimir Zadorozhny vladimir@sis.pitt.edu Information Science Program School of Information Sciences, University of Pittsburgh 1 Database: a Set of Relations

The Entity-Relationship Model 221 After completing this chapter, you should be able to explain the three phases of database design, Why are multiple phases useful? evaluate the significance of the Entity-Relationship

Physical Database Design Process Physical Database Design Process The last stage of the database design process. A process of mapping the logical database structure developed in previous stages into internal

The Entity- Relationship Model R &G - Chapter 2 A relationship, I think, is like a shark, you know? It has to constantly move forward or it dies. And I think what we got on our hands is a dead shark. Woody

Dening a Database Schema CREATE TABLE name èlist of elementsè. Principal elements are attributes and their types, but key declarations and constraints also appear. Similar CREATE X commands for other schema

C H A P T E R4 Intermediate SQL Practice Exercises 4.1 Write the following queries in SQL: a. Display a list of all instructors, showing their ID, name, and the number of sections that they have taught.

Objectives: CPS221 Lecture: Relational Database Design last revised June 21, 2011 1.To introduce the notion of primary and foreign keys 2.To introduce the anomalies that result from redundant storage of

GROUP 2 PRACTICE EXAMPLES FOR THE REVIEW QUIZ: Review Quiz will contain very similar question as below. Some questions may even be repeated. The order of the questions are random and are not in order of

Instant SQL Programming Joe Celko Wrox Press Ltd. INSTANT Table of Contents Introduction 1 What Can SQL Do for Me? 2 Who Should Use This Book? 2 How To Use This Book 3 What You Should Know 3 Conventions

CS27020: Modelling Persistent Data WORKSHEET: INTEGRITY CONTROL IN RELATIONAL DATABASES Time allowed: 40 minutes Calculators are not allowed in this worksheet. Answer all questions 1. Briefly explain what

Database Design Goal: specification of database schema Methodology: Use E-R model to get a high-level graphical view of essential components of the model and how they are related Convert E-R diagram to

Database Systems I (Compulsory) INTRODUCTION This is one of the 4 modules designed for Semester 2 of Bachelor of Information Technology Degree program. CREDITS: 04 LEARNING OUTCOMES On completion of this

SQL Server An Overview SQL Server Microsoft SQL Server is designed to work effectively in a number of environments: As a two-tier or multi-tier client/server database system As a desktop database system

UNIT 6 Structured Query Language (SQL) Text: Chapter 5 Learning Goals Given a database (a set of tables ) you will be able to express a query in SQL, involving set operators, subqueries and aggregations

Chapter 10, Functional Dependencies and Normalization for Relational Databases We need some formal measure of why the choice of attributes for a relation schema may be better than another. Functional dependencies

Using Temporary Tables to Improve Performance for SQL Data Services 2014- Informatica Corporation. No part of this document may be reproduced or transmitted in any form, by any means (electronic, photocopying,